1. Field of the Invention
This invention relates to a rotary head apparatus used for helically scanning a magnetic tape for recording and reproduction of signals, and more particularly is directed to such an apparatus that has magnetic heads with gaps at different azimuth angles.
2. Description of the Prior Art
It is known to record information by helically scanning a magnetic tape. For example, a conventional video tape recorder (VTR) that records and/or reproduces a video signal on a magnetic tape is an example of a helical scan type of recorder. In such recorders, it is well known to have two magnetic heads mounted at diametrically opposed positions on a rotary drum for recording respective signal channels. The drum may rotate at a frequency that is equal to the video frame rate. Typically the magnetic tape is obliquely wrapped around the peripheral surface of the rotary drum at an angle which may be, for example, slightly larger than 180.degree.. The two heads alternatively scan the magnetic tape, sequentially forming oblique tracks on the tape as the tape is advanced. The tape is advanced in a longitudinal direction and the oblique tracks are at an angle, known as a "track angle", to the longitudinal direction. A common track angle is about 5.degree.. In this conventional VTR, the video signal for one video field is recorded on each track and includes a frequency modulated luminance signal-and down-converted chrominance signals.
In a known type of VTR, a guard band, which is an area in which no signal is recorded, is provided between adjacent tracks in order to minimize or prevent crosstalk between the adjacent tracks. However, in order to extend the recording capacity of the tape, another type of VTR has been developed in which guard bands have been eliminated and a so-called "azimuth recording" approach is used to suppress crosstalk. In this approach the magnetic heads are arranged so that the head gaps are directed at angles to the head scanning direction. The angle by which the head gap is diverted from the scanning direction is known as the azimuth angle, and it is known to arrange the head gaps to have respective azimuth angles of opposite polarity and the same magnitude. For instance, in a known VTR using 8 mm tape, one head has an azimuth angle of 10.degree. while the other has an azimuth angle of -10.degree.. As a result, so-called "azimuth loss" reduces the crosstalk between adjacent tracks.
It is also known to form a magnetic tape by coating a base film with magnetic particles carried in a binding material. In order to increase the quantity of particles bound onto the base film, it is known to magnetically orient the particles during manufacturing so that the direction of the major axis of each of the particles is aligned in parallel with the longitudinal direction of the tape. As a result, the longitudinal direction of the tape is also a direction of magnetic orientation of the particles carried thereon. The direction of orientation of the particles is also referred to as the direction of magnetic orientation of the tape. When a rather small track angle, such as 5.degree., is used in forming the recording tracks, the deviation between the track direction and the tape's direction of magnetic orientation is small enough that the recording characteristics of the tape are not significantly affected by such deviation.
However, a small track angle, such as 5.degree., results in recording tracks that are rather long and which therefore are prone to non-linearity due to disturbances in tape transport speed, drum rotation speed, drum eccentricity and the like. Non-linearity of the recording tracks is particularly disadvantageous when it is desired to use a relatively narrow track. Therefore, it is desirable to increase the track angle, for instance to 10.degree., so that the length of the track is shortened and the susceptibility to non-linearity reduced. It will be appreciated that using a shorter track requires that the signal for one video field be divided or segmented among a plurality of tracks. However, distribution of a signal for one field among a plurality of tracks is well known and can be readily accomplished. An example of a recording format in which signals for one field are distributed among a plurality of tracks is the 4:2:2 (or D1) format shown at-pages 55 and 56 of Introduction to the 4:2:2 Digital Video Tape Recorder, by Stephen Gregory (Pentech Press, London, 1988).
Although the increase in the track angle provides advantages in terms of shortening the recording track and so reducing problems caused by non-linearity in the tracks, at the same time the increased track angle entails disadvantages due to the increased divergence between the scanning direction and the tape's direction of magnetic orientation. In particular, when heads having symmetrically inclined azimuth angles of opposite polarities are used, the deviation of the recording track from the direction of magnetic orientation results in asymmetry in the reproduction outputs provided by the respective magnetic heads. Taking the track angle to be .theta. and the respective azimuth angles to be .alpha. and -.alpha., it will be recognized that the angles of inclination of the heads with respect to the tape's direction of magnetic orientation are .theta.-.alpha. and .theta.+.alpha., respectively, and are not symmetric. Further, when a magnetic tape of the evaporation deposition type is used rather than the conventional coated tape, the magnetic orientation of the tape is stronger, which enhances the asymmetry of the outputs of the respective heads due to the asymmetric inclinations from the direction of magnetic orientation. As a result, there is dissidence between the reproduction outputs of the respective heads, and the quality of reproduction deteriorates because of noise, flickering and the like.